As to the conventional rotary switches, R.O.C Patent Publication No. 252616 discloses an “improved rotary switch structure”, wherein a rotary axle is installed inside a casing; the upper end of the rotary axle is pressed to engage with an operational knob; an electrically-conductive element is also pressed to engage with the rotary axle at an appropriate position near the lower end thereof; the lower end of the rotary axle is also engaged with a disc, which can be fixed to the casing with screws; a common terminal and multiple contact terminals are installed at appropriate positions of the disc; two protrusions of the electrically-conductive element can respectively contact the common terminal and one of the contact terminals. When such a rotary switch is used to control the rotation speed of an electric fan, a power wire is connected to the common terminal, and the wires for controlling different rotation speeds (such as low, middle, and high speeds) are respectively connected to those contact terminals of the disc. The user rotates the operational knob, and the operational knob drives the rotary axle to actuate the protrusions of the electrically-conductive element to respectively contact the common terminal and one of the contact terminals of the disc. Thus, the electric fan is turned on, and the rotation speed thereof is also determined. However, the abovementioned rotary switch has the following disadvantages:    1. The connection between the upper end of the rotary axle and the operational knob is implemented with merely a press engagement and lacks a secured mechanism. Between the operational knob and the rotary axle may exist a gap, which may deviate the operational knob and cause that the user cannot rotate the operational knob or the rotary switch may output a wrong instruction.    2. The press action may distort or fracture the operational knob, and may also damage the internal electrically-conductive element.    3. After a long-term usage, abrasion will enlarge the gap and loosen the engagement between the rotary axle and the operational knob. Therefore, when the user rotates the operational knob, the operational knob is likely to rotate ineffectually; thus, the rotary axle may be incorrectly actuated, and a wrong instruction may be created.
Therefore, the conventional rotary switch still has much room to improve.